1. Field of the Invention
This invention relates to semiconductor diode lasers and, more specifically, to fitting a long cavity laser in a compact chip using total internal reflection surfaces formed through etched facets.
2. Description of the Related Art
Semiconductor lasers typically are fabricated on a wafer by growing an appropriate layered semiconductor material on a substrate through Metalorganic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy (MBE) to form an epitaxial structure having an active layer parallel to the substrate surface. The wafer is then processed with a variety of semiconductor processing tools to produce a laser optical cavity incorporating the active layer and incorporating metallic contacts attached to the semiconductor material. Laser facets are typically formed at the ends of the laser cavity by cleaving the semiconductor material along its crystalline structure to define edges, or ends, of the laser optical cavity so that when a bias voltage is applied across the contacts, the resulting current flow through the active layer causes photons to be emitted out of the faceted edges of the active layer in a direction perpendicular to the current flow. Since the semiconductor material is cleaved to form the laser facets, the locations and orientations of the facets are limited; furthermore, once the wafer has been cleaved, typically it is in small pieces so that conventional lithographical techniques cannot readily be used to further process the lasers.
The foregoing and other difficulties resulting from the use of cleaved facets led to the development of a process for forming the facets of semiconductor lasers through etching. This process, as described in U.S. Pat. No. 4,851,368, also allows lasers to be monolithically integrated with other photonic devices on the same substrate, the disclosure of which is hereby incorporated herein by reference. This work was further extended and a ridge laser process based on etched facets was disclosed in the IEEE Journal of Quantum Electronics, volume 28, No. 5, pages 1227-1231, May 1992, the disclosure of which is hereby incorporated herein by reference.
The formation of total-internal-reflection (TIR) facets within an optical cavity through the use of etched facets at angles greater than the critical angle for light propagating within the cavity was also disclosed in U.S. Pat. No. 4,851,368 for broad area lasers and U.S. Pat. No. 5,031,190 for ridge lasers, the disclosures of which are hereby incorporated herein by reference.
High power semiconductor lasers are of significant interest for many applications, such as optical storage applications. As the power requirements for semiconductor lasers has increased, manufacturers have simply increased the cavity length of the laser chip, as shown in the graph of FIG. 1 from U.S. Patent Application 2006/0274802, where chip lengths are over 2000 μm long for 400 mW power required for double layer DVD applications. US Patent Application 2006/0274802, the disclosure of which is hereby incorporated herein by reference, states: “high power of a laser is demanded to improve a writing speed of an optical recording disk and therefore, an increase in laser resonator length is absolutely necessary to attain the high power. In this case, there is a problem in that the increase in chip size incurs a rise in chip cost.” This patent application also introduces a tapered multimode interference (MMI) waveguide in the middle of a ridge laser diode allowing the chip length to be reduced to 1300 μm for 300 mW output, resulting in a chip length that is about two-thirds of the chip length for a standard ridge laser (without the tapered MMI).
Early efforts using total internal reflection (TIR) facets included the following. “Rectangular and L-shaped GaAs—AlGaAs lasers with very high quality etched facets,” Applied Physics letters, Vol. 54, page 493, used a 45° TIR etched facet to demonstrate broad area L-shaped laser. “Continuous-wave operation and mirror loss of a U-shaped GaAs/AlGaAs laser diode with two totally reflecting mirrors,” Applied Physics letters, Vol. 56, page 1617, described a broad-area laser with two TIR facets, using cleaved front and back facets. “CW operation of folded-cavity semiconductor lasers with etched turning mirrors,” Electronic Letters, Vol. 28, page 21 described a folded cavity using ridge lasers of 80 and 5 μm ridge width with a cleaved back facet. “Precise determination of turning mirror loss using GaAs/AlGaAs lasers with up to ten 90° intracavity turning mirrors,” IEEE Photonics Technology Letters, Vol. 4, page 24, used wide ridge lasers with 10 μm ridge width with cleaved front and back facets.
The market continues to demand increased numbers of chips from a given wafer for variety of high power applications, such as DVD and Blu-ray read/write systems.